5.0 CONCEPTUAL DEVELOPMENT PLAN
PAH has based a conceptual development plan on two major principals. First, although there are important proven and probable ore reserves, significant time and expense will be required to develop this to the production stage. There are, fortunately, sufficient developed reserves in the Tajo to allow enough time to proceed with the orderly development of these new mining areas. Additionally, there is good reason to believe that new reserves will be found through additional exploration and effectively extend the mine life.
The second major principal is that Tintaya, like any operating mine, must continually strive to lower its cost structure. Although its current cost of US$0.58 places it at the bottom half of the cost curve, the introduction of new processing technology can have a significant positive effect.
The conceptual development plan is presented as a series of incremental improvements upon a base case. These improvements represent the introduction of new technology and result in lower unit costs. The base case is continued concentration of sulphide ore. Mining is according to the plan presented in Section 2.2 The first incremental improvement (Case 2) is the introduction of acid leaching and SX/EW recovery of copper oxide resources. The second improvement (Case 3 ) is the construction of a submerged combustion smelter used together with the concentrator and SX/EW plant. Cases 2 and 3 are based on the same mine plan as Case 1.
The evolution of these process alternatives are discussed in Section 5.1 (below). The three cases are more fully explained in Section 5.2.
5.1 Processing Alternatives - Tintaya
A number of possible alternatives exist whereby additional production might be achieved or value added by producing and shipping metal rather than concentrates.
This section outlines the possible process routes and costs for on-site metal production from concentrates, reviews past investigative work on additional production of metallic copper from oxide ores and examines the synergy between the two.
The combined costs of freight, smelting and refining of concentrates constitute approximately 45 percent of the total costs of copper production. The production of metal, in the form of blister or anode copper, at the mine site presents the potential for savings in overall production costs. The viabilility of on-site metal production will be, to a considerable extent, dependant on the operating life of the facilities and the rate at which they may be amortized. This, in turn, is dependant on the reserve base of the area. It is possible that reduction in operating costs by on-site metal production will positively affect cut off grades and therefore reserves. In addition, some of the processing alternatives considered for the production of metal from sulphide concentrates also present opportunities for low cost sulphuric acid production, which may substantially improve the cost of copper production from oxide reserves.
Traditional pyro-metallurgical methods of metal production from sulphide concentrates demand substantial economies of scale for viability; the current concentrate production rate at Tintaya and the probable constraints of future underground mining methods indicate that the economies of scale required for traditional smelting methods will not be achieved. However, emerging pyro-metallurgical techniques have been developed to the stage that relatively low cost metal production at rates comparable to those anticipated at Tintaya has been achieved. In addition, well established combinations of pyro and hydrometallurgical processes are possibly applicable to Tintaya concentrates, while less developed hydrometallurgical processes are worthy of consideration. The following list, although not exhaustive, summarizes potentially applicable processes:
Continued production and shipping of concentrates for custom smelting.
On-site flash smelting of concentrates.
Submerged combustion smelting of concentrates (known commercially as Sirosmelt, Ausmelt or Isasmelt).
Roasting and sulphuric acid leaching of concentrates.
Ammoniacal leaching of concentrates.
Bacterial leaching of concentrates.
Nitric/sulphuric acid leaching of concentrates.
Continued production and shipping of concentrates is used as a base against which to evaluate other technologies. Consideration of flash smelting technology uses the 1987 report by COMMSA. Submerged combustion smelting is a technology in which very high rates of reaction are achieved in a small, low cost smelting vessel by introducing fuel and either air or enriched air below the surface of a molten mass by means of a lance. Roasting and subsequent leaching of calcine has been used successfully at a number of operations world wide. Ammoniacal leaching has been successfully used in the past; response is mineral specific, it being particularly appropriate for chalcocite. Energy usage for this technology may be excessive. Bacterial leaching of sulphide concentrates has been successfully applied in the gold industry; although probably an inherent aspect of dump leaching of mixed oxide and sulphide ores, little experience is available in the base metal industries of concentrate leaching by this method. Nitric/sulphuric acid systems have been developed, though not applied, in the gold mining industry for the oxidation of sulphides, using the principle of nitric acid as a regenerable oxidant allowing the oxidation of sulphides at ambient temperatures in the presence of sulphuric acid. The principle was developed from patents specific to the oxidation of copper sulphides, but has not been developed within the copper industry.
Of the above processes, bacterial leaching and nitric/sulphuric acid systems are considered unproven at the industrial scale at this time, while ammoniacal leaching is unlikely to be appropriate for the suite of minerals in the Tintaya concentrates. Detailed review of technologies for the production of metal is, therefore, limited to submerged combustion technology, roast/leach systems, flash smelting and custom smelting.
5.1.1 Concentrate Production, Custom Smelting
This alternative represents continuation of current practice for realization of metal values in concentrates, and is used as a basis against which the potential advantages of other alternatives may be judged. No capital cost is incurred for this alternative, and operating costs may be expected to remain essentially constant. Transport and treatment charges may increase as Chabuca ores are treated, reflecting the lower bornite content of ores.
5.1.2 Flash Smelting of Concentrates
Flash smelting represents the most efficient of current conventional smelting methods. The use of oxygen enrichment allows fast reaction times for melting of sulphides and matte formation, while the minimization of off-gas quantity reduces the power requirement for gas handling and the corresponding enrichment of off-gases in SO2 allows the efficient scrubbing or production of sulphuric acid from those gases.
A study of the costs of flash smelting at the Tintaya site was carried out in 1987. This is sufficiently recent for the study results to be considered still valid, subject to
updating of costs. This study is therefore used as the basis for consideration of this technology.
A flash smelting furnace consists essentially of a reaction shaft and a containment for molten products; sulphides are rapidly oxidized while in suspension in the oxidizing gasses, which consist largely of oxygen enriched air. A degree of enrichment is usually adopted that allows the reaction to be autothermal. Indirect cooling of the reaction shaft is necessary. After separation from slag by difference in specific gravity, molten matte is tapped and transported to conventional converters for further oxidation to blister copper. A typical Flow sheet is shown in Figure 5.1.2-1
Although the reaction is rapid, the combined reaction shaft/molten product bath requires a massive structure, which, with the combined capital costs of oxygen plant and converters, leads to a highly capital intensive process. The unit capital cost per ton of capacity increases as the capacity decreases; for a relatively modest throughput such as that envisaged at Tintaya, the economics of flash smelting are marginal.
The capital cost of a flash smelter, situated at Tintaya, including all ancilliary plant and equipment is drawn from the COMMSA report of 1987 and escalated to 1993 costs at 5 percent annually as follows:
U.S.$x1,000
Smelter plant 59,886
Oxygen Plant 12,261
Sulphuric Acid Plant 14,304
Civil Works 1,986
Power, water etc. 2,462
E.P.C.M., patent fees etc. 11,816
Contingencies @ 7% 7,190
Working Capital 2,910
Interest Charges 12,675
Total 125,490
Figure 5.1.2-1
Annual operating costs are also drawn from the same report, and are either re-estimated at current costs or escalated at the same rate as capital costs.
U.S.$x1,000
Supervision 233
Labor 1,041
Maintenance 5,490
Consumables: Fuel 9,900 T@$339/T3,356
Power 50 GWh @$0.01/kWh 5,000
Silica flux 18,150 T @$9/T 163
Refractories 215 T@$2,000/T 430
Process water 120
Propane 395
Miscellaneous 10% 1,352
Total 17,580
5.1.3 Submerged Combustion Smelting
Submerged combustion smelting is an emerging technology that has been successfully applied at an industrial scale and has shown potential savings in capital costs relative to flash smelting for small to medium sized operations. Unlike a flash smelter, in which fast reaction kinetics are achieved by oxidation of sulphides in gas suspension, the sulphides are introduced, together with fuel and air or oxygen enriched air, below the surface of a pool of molten matte and slag. This is achieved by means of a lance that is protected by frozen slag. The resulting reactions are not only extremely fast, but do not require the volume of the reaction shaft of the flash furnace. Relatively small, low cost furnaces are therefore required. A typical Flow sheet is shown in Figure 5.1.3-1.
Figure 5.1.3-1
It is possible to use a single vessel for both smelting, which is a continuous process and converting, which is a batch process. However, this gives rise to great fluctuations in gas flows. If a sulphuric acid plant is operated, better control of gas flows is obtained by operating separate vessels or conventional converters. Flow of matte from smelting to converting vessel may be in liquid or granulated form.
Enrichment of air by gaseous oxygen may be advantageous but is not necessary even at the altitude of Tintaya. The major advantages of oxygen enrichment are a smaller furnace, lower fuel consumption, lower off-gas volumes and off-gas richer in SO2 . The major disadvantages are the high capital cost of the oxygen plant and the power consumption required for oxygen production. This study addresses smelting with and without oxygen enrichment, but this subject requires study in much greater detail.
Submerged Combustion Smelting without Oxygen Enrichment
This process alternative requires two submerged combustion furnaces, each 5.5 m outside diameter by 8 m high. The smelting furnace would operate continuously, while the converting furnace would operate on a batch basis, the batch cycle approximating the output of the smelting furnace. Surge capacity between the two and independence of operation would be provided either by granulating the matte or by means of a settling and storage furnace. The smelting furnace would accept feed of concentrates at the current moisture content of 10 percent and produce a matte of approximately 60 percent Cu, with 2 to 3 percent loss of total copper to slag. For the purpose of this study, slag losses are considered total, but it would be possible to recover slag losses by grinding and floating matte from slags. The converter furnace would produce a blister product consisting of 97-98 percent copper and containing a high percentage of the precious metal values. Converter slag, averaging 5 to 8 percent copper, would be recycled to the smelter furnace.
Off-gas from both furnaces would be cooled and cleaned in separate streams, then combined and fed to an acid plant. The combined off-gas average flowrate would be of the order of 55,000 Nm3/hr and contain 7 to 10 percent SO2. This is rather low for conventional acid production, and fluctuations in both quantity and tenor of the gas due to intermittent converter operation will make achievement of consistent acid strength difficult. Production of acid for local use or disposal will be possible, but it is doubtful that a saleable acid product could be produced.
Operating consumables would consist largely of fuel, power, fluxes and refractories. It is likely that some coal would be required as a reagent, which would displace an equivalent amount of liquid fuel. Manning requirements would be of the order of 12 staff and 60 hourly paid labourers.
Submerged Combustion Smelting with Oxygen Enrichment
Oxygen enrichment could be implemented over a range of feed gas oxygen contents ranging from ambient to limits imposed by control of temperature and smelter products. As oxygen enrichment is increased, fuel requirement decreases, furnace size and off-gas volumes decrease while the size and operating cost of an oxygen plant increase. The degree of oxygen enrichment required, if any, requires detailed study. For the purposes of this study, enrichment to 30 percent oxygen in feed gas is considered, requiring the production of 125 tpd of 95 percent gaseous oxygen. Furnace size would be reduced to 3 m dia. off-gas volumes would be reduced by 30 percent to 35 percent and SO2 content increased to between 14 and 17 percent. Fuel requirements would be reduced by 45 percent.
Installed capital costs of the two options, based on conceptual estimates and a two year construction period, are compared below:
Without O2 With O2
U.S.$x1,000 U.S.$X1,000
Submerged combustion furnace 8,000 6,000
Feed handling, gas handling, matte transfer etc. 23,000 22,000
Acid plant 18,000 15,000
Oxygen plant --- 12,000
E.P.C.M. 5,000 6,000
Contingency 5,000 6,000
Interest during construction 4,400 5,000
Totals 63,400 72,000
Annual operating costs are similarly compared below:
Without O2 With O2
U.S.$x1,000 U.S.$X1,000
Supervision 140 150
Labor 560 675
Maintenance 3,000 4,000
Consumables: Fuel No 2 diesel @ $339/T 5,085 2,034
Coal @ $100/T 500 500
Power @ $0.01/kWh 2,000 7,200
Silica & limestone flux 25,000T @$8/T 200 200
Refractories @$2,000/T 1600 600
Process water 120 120
Contingency1,000 1,000
Total 13,165 16,479
5.1.4 Roast/Leach/Electrowin
The principle of this process route is to render copper sulphide minerals acid soluble by oxidation. Oxidation is achieved by roasting at a temperature sufficient to cause oxidation, but not high enough to cause fusion. Sulphuric acid for use in leaching of the resulting oxides may be produced from the resulting off-gases or provided from an external sources. Copper in solution as sulphate may be recovered by electro winning or solvent extraction and electro winning.